`shown` read as the shape on screen. It is the shapes decoded and given
to the UI, keyed by content id, so a shape arriving under a new handle
is looked up once instead of scanning the handle map: `decoded`,
`has_decoded`, and `record` for the call that fills both maps. The web
mirror is renamed the same way.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab